As a high-density and large-capacity recording medium, a diversity of optical disks including digital audio disks, video disks, and CD-ROMs are widely used. Now a high-density and large-capacity digital video disk (DVD) having a capacity (4.7 GB) seventh times more than a CD-ROM (with a memory capacity of 640 MB) is already commercially available. In optical memory technology underlying such disks, information is recorded and reproduced using a laser beam L which is focused by an objective lens 109 to a definition on the order of several micrometers as shown in FIG. 5. In information reproduction, the laser beam L is directed to an optical disk 103, and the reflected laser beam L is detected by an optical sensor 117 via a beam splitter 115. Shown further in FIG. 5 are a light source 105 and a diffraction grating 113.
In such an optical information recording and reproduction apparatus, the diameter of a small spot projected on the optical disk should be as small as possible to achieve a high-density and large-capacity goal. The size of the spot on the optical disk or spot definition is determined by the wavelength of the laser beam and the numerical aperture of the objective lens. To reduce the diameter of the spot on the optical disk 103, design efforts are made to shorten the wavelength of the laser beam L and to increase the numerical aperture of the objective lens.
To meet a high-density requirement imposed on the optical disk, the wavelength of the laser beam L is shortened, and the numerical aperture of the objective lens is increased, and the diameter of the spot is actually reduced. On the other hand, focal depth becomes shallow, degrading tilt characteristics. More particularly, reproduction performance in reproducing information is not reliably maintained due to the mechanical inclination of the optical disk with respect to the objective lens (warp) and the runout of a spindle motor (a displacement from a plane perpendicular to the axis of rotation).
To resolve the above problem, thinning the thickness of the optical disk may be contemplated. In a high-density optical disk, the problem is resolved by thinning the thickness of the optical disk and by implementing an optimum wavelength of the laser wavelength and an optimum numerical number of the objective lens in design. If the optical head device having the optimum design adapted to a thin optical disk is used with optical disks of a conventional thickness, a problem will arise. When a spot of the laser beam is projected onto the optical disk, a spherical aberration takes place due to the differences in the thickness of the optical disk, and highly reliable information reproduction cannot be performed.
FIG. 6 shows the relationship between an objective lens and the thickness of an optical disk. As shown in FIG. 6(a), for example, the objective lens is designed to be adapted to a thin optical disk 103A. A spot subject to diffraction limited is produced on a storage layer 103a of the optical disk 103A.
Referring to FIG. 6(b), an objective lens 109 adapted to a thin optical disk is used with a thick optical disk 103B. Unlike the case shown in FIG. 6(a), a spherical aberration takes place due to the difference in thickness of the optical disk 103B, and the intended spot diameter cannot be achieved. Reliable reproduction of the recorded information is rendered impossible.
One of the solutions to this problem is to vary the aperture of an objective lens. As aperture varying means, a liquid-crystal display element is used. More particularly, aperture varying means is installed upstream of the objective lens of the optical head device and an optical sensor is also provided. Depending on the thickness of the optical disk, the diameter of the laser beam incident on the objective lens is varied by the aperture varying means. Since the diameter of the laser beam is merely varied in such a method, aberration correction to the objective lens is not sufficient enough, leaving recording and reproduction characteristics unimproved.